You'll have all heard the acronym, LED, because it's everywhere these days, on TVs, tablets, you name it. But the acronym HID was unfamiliar to me, because I don't have much to do with cars in terms of doing anything more than driving them. I've never been someone who's accessorized his vehicle, so the car we have is the car we got. No frills have been attached.
Anyway, HID stands for 'high-intensity discharge' in relation to lamps and headlights, and no doubt various other things that lighten up your particular world. Wikipedia tells me that these lights function by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. There you go. You're much wiser now. (I'm still having to check out what the various words mean that go to make up the explanation.)
The place many people will come across these lights are on cars - I suspect they're those ones that dazzle you as meet them face-to-face, or headlight-to-headlight, on the road. They're no doubt useful, but not when they're blinding the drivers of oncoming traffic.
And it's wise, I understand, to use proper HID headlight instructions when you're installing them - if you're the sort of person who does this kind of at-home installation. (I've already told you I'm not.) The manufacturers are not liable for you putting them in backwards, sideways or any other way that isn't appropriate, or wiring up the wrongs bits of wire and setting your car on fire. So you've been warned. Do it properly, or else get a professional.
Today's bit of useful advice from me.